Obesity is highly prevalent in American Indian children, increasing their risk for the development of cardiovascular disease, hypertension and diabetes. Although increasing energy expenditure, through increasing physical activity, and decreasing dietary fat intake should slow the rate of body fat gain, it appears that children actually decrease their physical activity as they mature from pre-to post-pubescence. Several studies have shown that the decline in activity is greater in females than in males, and greater in non-whites than in whites; however, other studies have not supported these findings. Further, there are no data available in American Indian children, which is a population at greatest risk. Additionally, inconsistencies have been reported with respect to the relationship between body weight or body fat and energy intake or dietary fat intake. At the heart of the problem is the difficulty in measuring self-reported physical activity and dietary intake accurately. Accurate self- report measures are necessary for use in the large scale studies that are required to investigate relationships between physical activity and dietary intake and disease. The Healthy People 2000: National Health Promotion and Disease Prevention Objectives include seven objectives emphasizing increasing physical activity in children and adolescents. In support of Healthy People 2000, the National Institutes of Health: National Heart, Lung and Blood Institute has published a Program Announcement (PA-95- 004)entitled, Physical Activity and Cardiopulmonary Health, to solicit well-defined studies in the area of physical activity related to cardiovascular disease risk factors. This study is a combined longitudinal and cross-sectional design in which we will modify the best existing methods for the self-report of physical activity and dietary intake in children and adolescents, and evaluate their validity using total energy expenditure (TEE) measured by doubly labeled water (DLW) as the criterion. We will then use these methods to measure physical activity and dietary intake in two age cohorts of male and female American Indian children (8-10-years and 11-13-years) at baseline and two years later, with two sets of interim measurements. Change in TEE from baseline to the two-year follow-up will also be measured using DLW and the TriTrac accelerometer. Equations for the estimation of percent body fat will be cross-validated for this population. The proposed study will fill an existing gap by improving existing instruments for the measurement of physical activity and dietary intake in children and adolescents, and will provide longitudinal data on change in these variables in the same children over time, plus provide cross-sectional data for each age and gender group to compare to the existing literature.